The SA 508 Gr. 3 forged steels have been used for the reactor pressure vessels of pressurized light-water reactors and the pressurizer shells and steam generator shells of nuclear power plants.
The reactor pressure vessel materials need excellent properties such as high resistance against the embrittlement by fast neutron irradiation, high toughness, high fatigue life, high homogeneity, and good weldability because they are used for long terms over 40 years in the severe conditions of high temperature, high pressure and neutron irradiation.
Particularly, fast neutron irradiation in the belt-line region of reactor pressure vessel causes to decrease the upper shelf energy(USE) and to increase the ductile-to-brittle transition temperature(DBTT) during operating. This embrittlement phenomenon in the pressure vessel limits the operating conditions and the life of the power plant. Accordingly, it is preferable to manufacture the pressure vessel steels having high toughness in order to obtain the operating margins and to extend the life of the power plant.
Conventionally, the quality heat treatment, the heat treatment process after forging, of SA 508 Gr. 3 steel forgings is consisted of quenching, tempering and post-weld heat treatment, and the methods and the conditions of the heat treatment are specified in ASME/ASTM specification(ASME "Specification for Quenched and Tempered Vacuum-Treated Carbon and Alloy Steel Forgings for Pressure Vessels", ASME SA-508/SA-508M, 1995, pp. 785-792, ASTM "Standard Specification for Quenched and Tempered Vacuum-Treated Carbon and Alloy Steel Forgings for Pressure Vessels", ASTM A 508/A 508M-95, 1995, pp. 1-6).
The quenching treatment is to cool the material in water after annealing at a high temperature for austenitization. Therein the fine microstructure and high toughness of final product can be obtained by increasing the cooling rate and by optimizing the tempering condition.
However, the toughness of the pressure vessel steels can not be increased by a change in the conventional heat treatment processes because the pressure vessel is an extremely heavy component whose thickness usually reaches 10 inches or more, thus it is substantially impossible to increase the cooling rate up to over 30.degree. C. per minute.
ASME/ASTM specifies that the tempering of the SA 508 Gr. 3 steel is performed at over 650.degree. C. for over 30 minutes per one inch thickness of the vessel wall in order to obtain sufficient toughness; 10 inch wall needs a tempering for at least 5 hours.
As a complementary condition for the case that post-weld heat treatment is applied, ASME/ASTM specifies that the tempering can be performed at a temperature over 635.degree. C. It is, in general, performed at over 650.degree. C.
Meanwhile, the intercritical heat treatment(IHT) has been used in the processes for manufacturing dual-phase steel plates, especially for applications to automobile industry. High strength and ductility are obtained by dispersing the martensite phase, usually 5 to 40%, in ferrite matrix. The ferrite-martensite dual phase structure is obtained by the heat treatment process consisted of the intercritical annealing at the ferrite-austenite two phase region and quenching.
Recently, the intercritical heat treatment has been introduced to the manufacturing processes of the quenched and tempered steels such as 9Ni steels, rotor steels, pressure vessel steels, etc, to improve the toughness.
German Skamletz et al (T. A. Skamletz and W. W. Grimm, "Advanced Technology of Heavy-Section Tube Sheets for Nuclear Power Generation", Steel Forgings, ASTM STP 903, E. G. Nisbett and A. S. Melilli, Eds., American Society for Testing and Materials, Philadelphia, pp. 410-424, K. Forch, W. Witte, and S. H. Hattingen, "Application of Three-Stage Heat Treatment to Thick-Walled Workpieces from Weldable, High-Strength Fine-Grained Structural Steels and Reactor Steels", Stahl u. Eisen 100 (1980) 1329-1338, K. D. Haverkamp, K. Forch, K.-H. Piehl, and W. Witte, "Effect of Heat Treatment and Precipitation State on Toughness of Heavy Section Mn--Mo--Ni--Steel for Nuclear Power Plants Components", Nucl. Eng. & Design 81 (1984) 207-217) had reported that the three step heat treatment of DIN 20 Mn--Mo--Ni 55 steel, a similar steel to SA 508 Gr. 3 steel, including an additional annealing step at 750.degree. C. to 770.degree. C. between quenching and tempering can increase the impact energy and decrease the transition temperature. In their reports, however, the increase of impact energy has not been observed at the temperatures higher than room temperature
Nisbett(E. G. Nisbett: J. Eng. Mater. Technol. (Trans ASME), 100 (1978) 338-347) had reported that when SA 508 Cl. 2 steel is treated with intercritical annealing at 790.degree. C., the impact toughness value increases.
The ASME/ASTM specification(1995 edition) for SA 508 Gr. 3 steel forgings allows the re-austenitization treatment; an intercritical heat treatment can be performed by re-heating to the intercritical temperature region to partially reproduce austenite phase.
In order to improve the mechanical properties of the final product, it is important to optimize the volume fraction of austenite in the two phase region by controlling the temperature and time of the intercritical annealing.
However, the conditions of intercritical heat treatment are different from each other according to alloy systems; the condition for improving the impact properties of SA 508 Gr. 3 steels is not presented at the present time.
Under the above situation, we, the inventors of the present invention, have tried to establish the condition of intercritical heat treatment of SA 508 Gr. 3 steel in order to improve its toughness and have completed the present invention.